I don't know why 20 was popular but 12 is divisible by 2, 3, 4 and 6, and when multiplied by 5 equals 60, which is divisible by 20, of course. 30 is divisible b

I don't know why 20 was popular but 12 is divisible by 2, 3, 4 and 6, and when multiplied by 5 equals 60, which is divisible by 20, of course. 30 is divisible by 2, 3, 4 and 5 as well, but is not divisible by 20, so 60 was preferred, perhaps.